This invention relates to the preparation of both positive and negative electrodes for use in high-energy secondary electrochemical cells and batteries that can be employed as power sources for electric automobiles and for the storage of electric energy generated during intervals of off-peak power consumption. A substantial amount of work has been done in the development of such electrochemical cells and their electrodes. The cells showing the most promise employ alkali metals, alkaline earth metals and alloys of these materials as negative electrodes opposed to positive electrodes including the chalcogens and metal chalcogenides as active materials. Typical examples include lithium, sodium or calcium and alloys of these active materials with more stable elements such as aluminum and silicon as the negative electrode materials. In the positive electrode, active materials advantageously include metal sulfides and mixtures of metal sulfides such as the iron sulfides, cobalt sulfides, copper sulfides, nickel sulfides, and molybdenum sulfides.
Examples of such secondary cells and their components are disclosed in U.S. Pat. No. 3,907,589 to Gay et al., entitled "Cathodes for a Secondary Electrochemical Cell" and in allowed U.S. Pat. No. 3,947,291, Mar. 30, 1976, to Yao et al., entitled "Electrochemical Cell Assembled in Discharged State"; U.S. Pat. No. 3,933,521, Jan. 20, 1976 to Vissers et al., entitled "Improved Anode for a Secondary High-Temperature Electrochemical Cell"; U.S. Pat. No. 3,941,612, Mar. 2, 1976 to Steunenberg et al., entitled "Improved Cathode Composition for Electrochemical Cell"; and U.S. Pat. No. 3,933,520, Jan. 20, 1976 to Gay et al., entitled "Method of Preparing Electrodes with Porous Current Collector Structures and Solid Reactants for Secondary Electrochemical Cells". Each of these patents and patent applications is assigned to the assignee of the present application. In addition to these high-temperature cells, the present invention is also applicable to the more conventional lead-acid and nickel-cadmium cells.
Prior electrodes have been prepared by various techniques and many have performed reasonably well. A number of problems still exist respecting long-life electrodes having sufficiently high specific energy and specific power for such as vehicular applications. Active materials in solid rather than liquid form have been selected to enhance retention and cell life. However, the uniform distribution of active material within current collector structures without drifting during operation continues to be of concern.
In other electrodes, paste mixtures of molten-salt electrolyte and particulate active material have been pressed into electrically conductive metal screens, mesh or other lattice structures. These type electrodes are tedious to prepare, as they require elevated temperatures over extended periods of time during the pressing operation. Also, it has been difficult to form a uniform electrode with hot pressing techniques.
In other electrodes, particular active material has been vibrated into a porous electrically conductive current collector structure. In this method, the particle sizes and substrate interstices must be appropriately matched to obtain adequate loading and to prevent slumping of the material within the substrate. Such a vibratory loading technique can present problems where the active material undergoes substantial volumetric changes between the condition in which it is loaded and the conditions it attains during cycling. This, for example, occurs when iron sulfides react to form lithium sulfide.
One technique for obtaining uniform loadings of active material of about 30 to 40 percent of theoretical is to melt molten-salt electrolyte with the active material, e.g. FeS or FeS.sub.2 particles, solidify and then regrind to obtain suitable material. Although such material can be loaded uniformly by vibrating methods into a porous substrate, the active material may well slump during cell operation when the electrolyte again becomes molten.